Method of operating a liquid-gas electrochemical cell

ABSTRACT

The invention includes in its scope a method for electrochemically reacting a liquid with a gas in an electrochemical cell of the type having at least two electrodes separated by a liquid permeable separator. At least one of said electrodes is in physical contact with the separator and is porous and self-draining. A gas is flowed into at least a portion of the pores of the self-draining electrode and a liquid is controllably flowed through the separator and into the self-draining electrode at a rate about equal to the drainage rate of the electrode and in a quantity sufficient to fill only a portion of the electrode pores. The liquid and the gas are electrochemically reacted to form at least one nonvolatile product. Thereafter, the products of the electrochemical reaction are removed from said self-draining electrode.

This application is a continuation-in-part of the copending applicationSer. No. 349,891, filed Feb. 18, 1982, now U.S. Pat. No. 4,406,758.

This invention is an improved method of operating an electrochemicalcell.

BACKGROUND OF THE INVENTION

Gas electrodes, for example oxygen electrodes, are well known in the artand are useful in many processes including chlor-alkali processes andprocesses for the production of hydrogen peroxide. Oxygen electrodes aregenerally porous. In such electrodes, reactions occur at the point(s)where there is a three-phase contact between a gas, an electrolytesolution and a solid electrical conductor. To maximize the efficiency ofthe electrode, the amount of the three-phase contact area should bemaximized. If the electrode is filled with the electrolyte, the rate ofmass transfer of gas to the electrical conductor is too slow to besignificant and is therefore not useful. Conversely, if the electrode isfilled with gas, the absence of the electrolyte solution allows onlyinsignificant amounts of reactions to occur.

Packed bed electrolytic cells of the type described in U.S. Pat. Nos.3,969,201 and 4,118,305 are commonly used. A porous separator separatesthe packed bed electrode from the adjoining electrode and is supportedby the packed bed electrode. The pores of the separator are sufficientlylarge to allow free flow of electrolyte into the openings of the packedbed electrode. Electrochemical reactions occur within the electrode at agas-electrolyte-electrode interface. The liquid products and unreactedelectrolyte flow by gravity to the bottom of the packed bed electrode.Mass transfer is a problem in such cells because the electrode is almostflooded with electrolyte. Reactions are slow and recycle of the productis necessary.

SUMMARY OF THE INVENTION

The invention includes in its scope a method for electrochemicallyreacting a liquid with a gas in an electrochemical cell of the typehaving at least two electrodes separated by a liquid permeableseparator. At least one of said electrodes physically contacts theseparator and is porous and self-draining. A gas is flowed into at leasta portion of the pores of the self-draining electrode and a liquid iscontrollably flowed through the separator and into the self-drainingelectrode at a rate about equal to the drainage rate of the electrodeand in a quantity sufficient to fill only a portion of the electrodepores. The liquid and the gas are electrochemically reacted to form atleast one nonvolatile product. Thereafter, the products of theelectrochemical reaction are removed from said self-draining electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of the invention in an electrolyticcell which has a diaphragm-type separator.

FIG. 2 shows another embodiment of the invention. Illustrated is anelectrolytic cell which has an ion exchange membrane in addition to adiaphragm-type separator.

DETAILED DESCRIPTION OF THE DRAWINGS

In the illustrated embodiment of the invention, FIG. 1 shows anelectrolytic cell 100. The cell has an anode 120 which is located in ananolyte chamber. An electrolyte inlet port 116 opens into the anolytechamber. A gaseous product outlet port 122 is located in the anolytechamber.

The cathode 106 is an electrically conductive porous mass having aplurality of pores passing therethrough. It may be a bed ofelectroconductive sintered particles or an agglomeration of looseparticles. It must have pores of sufficient size and number to allowboth gas and liquid to flow therethrough. The pores must also be of asufficient size such that nonvolatile products will flow by gravity tothe lower portion of the cathode 106, i.e., the cathode should be"self-draining". Another way of expressing this is to describe the poresas being large enough so that gravity has a greater effect on the liquidin the electrode than does capillary pressure.

Optionally, the cathode may be supported in some manner. For example,FIG. 1 shows a porous screen support 102 for the cathode.

A diaphragm-type separator 112 is positioned against the cathode 106 orthe cathode support 102. It may be a mass of tightly packed fibers suchas asbestos or fluorocarbon fibers which may be woven or merelyagglomerated in a random fashion. The separator 112 may be a pluralityof layers or a single layer. However, the separator material should besubtantially chemically inert to the chemicals that it will contactunder ordinary operating conditions. The separator is constructed sothat it has a somewhat limited ability to allow a liquid to flowtherethrough. Uncontrolled flow of liquid through the separator isintolerable in the present invention. The separator and electrode arepositioned to be in physical contact with each other, so that liquidsflowing through the separator flow immediately into the electrode. Thespace therebetween should be minimized to provide the most efficientcontrol of the amount of liquid flowing through the separator into theelectrode. The separator may be supported by the electrode, or it may besupported by a separate support element. Optionally, the separator maybe self-supporting.

FIG. 2 illustrates another embodiment of the invention. It shows anelectrolytic cell 101 having an ion exchange membrane 124. Thisembodiment operates in a manner quite similar to the cell illustrate inFIG. 1. Here, the ion exchange membrane acts as a barrier to control themigration of ions into and out of the cathode chamber 128.

In the embodiment of FIG. 2, the hydraulic pressure on the separator 112is controlled in the same manner as was discussed in relation to thecell in FIG. 1.

As with the embodiment in FIG. 1, the electrolyte which is in contactwith the separator must exert a hydraulic pressure on the separator. InFIG. 2, however, it is the electrolyte in the cathode chamber 128 whichexerts the critical hydraulic pressure. Thus, the method of controllingthe hydraulic pressure of the electrolyte on the separator refers to theelectrolyte in the cathode chamber 128, rather than to the electrolytein the anode chamber 118 of FIG. 1.

In the invention, liquid flow through the separator 112 should becontrolled at a level sufficient to fill only a portion of the pores inthe cathode 106. If too much liquid passes through the separator andsubstantially all of the pores of the cathode 106 are filled, thepresence of oxygen gas is minimized. This results in a very slowreaction to form the products of electrolysis. Conversely, if too littleelectrolyte passes through the separator 112 and into the pores of thecathode 106, the electrochemical reactions will be minimized. A criticalaspect of the present invention is to prevent the almost total fillingof the cathode pores while at the same time preventing the almost totalabsence of electrolyte from the cathode pores.

The volume of liquid flowing through a porous separator is thought to bedefined by the following equation: ##EQU1## where V=volume flow rate,cm³ /sec.

K=permeability, cm²

A=geometric area of the surface of of the separator contacted by theliquid, cm²

ΔP=pressure drop across the separator, g/cm sec²

μ=liquid viscosity, g/cm sec.

d=separator thickness, cm.

Generally, the viscosity (μ) of the liquid is constant and depends uponthe process in which the invention is used. The construction of theseparator determines its thickness (d) and its permeability (K).

Generally, diaphragms used in chlor-alkali electrolyte cells havepermeabilities (K) from about 1×10⁻⁸ to about 1×10⁻¹⁰ cm². Naturallythis varies with the variables in the equation shown above.

Thus, there are two convenient means for controlling the flow throughthe separator into the electrode. One way is by varying the area (A) ofthe separator contacted by the liquid and a second way is by adjustingΔP, the pressure drop across the separator.

A convenient way of controlling the area of the separator exposed to theliquid is by increasing or decreasing the height of the liquid reservoiradjoining the separator. As the height is increased, the flow throughthe separator increases. Conversely, as the height is decreased, theflow decreases.

The other method of controlling the flow through the separator is bycontrolling the pressure drop across the separator. This pressure dropmay be controlled in several ways.

One method of controlling the pressure drop across the separator is byoperating the chamber opposite the self-draining electrode under gas orliquid pressure. In this method, the opposing chamber is sealed from theatmosphere and gas pressure or liquid pressure is exerted on theelectrolyte. High pressure pumps may be used to force a pressurizedliquid into the opposing chamber or pressurized gas may be fed to thechamber.

Another method of controlling the pressure drop across the separator isby pulling a vacuum on the self-draining electrode side of theseparator. This will pull the electrolyte toward and through theseparator and finally into the self-draining electrode.

The herein described method may be used in any process in which a liquidis reacted with a gas. It is particularly useful in electrochemicalcells. It is particularly useful in cells used for the production ofhydrogen peroxide, for the production of chlorine and caustic and forthe production of energy (fuel cells).

Although the herein described method may be used in a variety ofelectrolytic processes, its use will be described for the production ofchlorine and caustic. In operation of the electrolytic cell 100, a NaClbrine solution is fed into the anode compartment through inlet port 116.The electrolyte contacts the anode 120 and the diaphragm-type separator112. Hydraulic pressure is exerted by the electrolyte upon the separator112.

An oxygen containing gas enters the porous, self-draining cathode 106through a gas inlet 104. The gas flows through the pores of the cathode106 where electrochemical reactions occur with the electrolyte. At leasta portion of the gas is consumed in such reactions to produce sodiumhydroxide. The liquid sodium hydroxide flows by gravity to the lowerportion of the cathode 106 and is removed through outlet port 108.

The hydraulic pressure of the electrolyte against the separator 112 iscontrolled at a level which will force the electrolyte to flow throughthe separator 112 and into the porous, self-draining cathode 106.

EXAMPLE

An aqueous slurry containing asbestos fibers was prepared. The slurrywas vacuum drawn through a porous plate and a substantial portion of theasbestos was thereby deposited on the porous plate. Asbestos was sodeposited until the asbestos layer of the separator had a thickness of1/8 to 1/4 inch.

The so-formed separator was subjected to a series of measurements todetermine the flow rate of a fluid through the separator at variousfluid head pressures.

The results were as follows:

    ______________________________________                                        Head Pressure Flow                                                            (psi)         (ml/sec-cm.sup.2)                                               ______________________________________                                        0.5           8.2 × 10.sup.-4                                           1.0           1.4 × 10.sup.-3                                           1.5           2.0 × 10.sup.-3                                           2.0           2.6 × 10.sup.-3                                           2.5           3.1 × 10.sup.-3                                           3.0           3.6 × 10.sup.-3                                           3.5           4.0 × 10.sup.-3                                           ______________________________________                                    

What is claimed is:
 1. A method for electrochemically reacting a liquidwith a gas in an electrochemical cell of the type having at least twoelectrodes separated by a liquid permeable separator; at least one ofsaid electrodes being in physical contact with said separator and beingporous and self-draining, said method comprising:(a) flowing a gas intoat least a portion of the pores of the self-draining electrode; (b)controllably flowing the liquid through said separator and into theself-draining electrode at a rate about equal to the drainage rate ofthe electrode and in a quantity sufficient to fill only a portion of theelectrode pores; (c) electrochemically reacting the liquid with the gasto form at least one nonvolatile product; and (d) removing theelectrochemical products from said self-draining electrode.
 2. Themethod of claim 1 including controlling the amount and rate of liquidflowing through the separator by adjusting the area of the separatorcontacted by the liquid.
 3. The method of claim 1 wherein the amount andrate of liquid flowing through the separator is controlled by adjustingthe atmospheric pressure exerted on the liquid.
 4. The method of claim 1wherein the nonvolatile product is sodium hydroxide.
 5. The method ofclaim 4 where the self-draining electrode is a cathode.
 6. The method ofclaim 1 wherein the gas is an oxygen containing gas.
 7. The method ofclaim 6 wherein the oxygen containing gas is air.
 8. The method of claim6 wherein the oxygen containing gas is substantially pure oxygen.
 9. Themethod of claim 6 where the self-draining electrode is a cathode. 10.The method of claim 1 where the self-draining electrode is a cathode.11. The method of claim 10 wherein the cell includes an ion exchangemembrane located between the separator and an anode.
 12. The method ofclaim 1 wherein the separator is self-supporting.
 13. The method ofclaim 1 wherein the separator is supported by a support element.